1. Field of the Invention
The present invention relates to an optical filter for compensating for color shift, and more particularly, to an optical filter, which is provided in front of a display panel to minimize color shift according to an increase in the viewing angle, and a display device having the same.
2. Description of Related Art
In response to the recent emergence of high-level information societies, components and devices related to image displays are being significantly improved and rapidly distributed. Among them, image-displaying devices to be used for televisions, monitors of personal computers, etc. are being widely distributed. In addition, there are attempts to enlarge the size while reducing the thickness of display devices.
In general, a Liquid Crystal Display (LCD) is one type of flat panel display that displays images using liquid crystal. The LCD is widely used throughout the industry since it has advantages such as light weight, low drive voltage, and low power consumption compared to other display devices.
FIG. 1 is a conceptual view schematically illustrating the basic structure and operating principle of an LCD 100.
For example, a conventional Vertical Alignment (VA) LCD includes two polarizer films 110 and 120, the optical axes of which are perpendicular to each other. Liquid crystal molecules 150 having birefringence characteristics are arranged between two transparent substrates 130, which are coated with transparent electrodes 140. When an electric field is applied from a power supply unit 180, the liquid crystal molecules move and are aligned perpendicular to the electric field.
Light emitted from a backlight unit is linearly polarized after passing through the first polarizer film 120.
As shown in the left of FIG. 1, liquid crystal remains perpendicular to the substrates when the power is off. The liquid crystal, in this state, can not have any effect on the polarization of the light. As a result, the light maintaining the linearly-polarized state is blocked by the second polarizer film 110, the optical axis of which is perpendicular to that of the first polarizer film 120.
As shown in the right of FIG. 1, when voltage is applied, the liquid crystal shifts to a horizontal position parallel to the substrates, between the two orthogonal polarizer films 110 and 120, in response to the electric field. Thus, the linearly-polarized light from the first polarizer film is converted into another linearly-polarized light, the polarization direction of which is perpendicular to that of the linearly-polarized light passing from the first polarizer film, circularly-polarized light, or elliptically polarized light while passing through the liquid crystal molecules just before it reaches the second polarizer film. The converted light is then able to pass through the second polarizer film. It is possible to gradually change the orientation of the liquid crystal from the vertical position to the horizontal position by adjusting the intensity of the electric field, and to thereby control the intensity of light emission.
FIG. 2 is a conceptual view illustrating the orientation and optical transmittance of liquid crystal depending on the viewing angle.
When liquid crystal molecules are aligned in a predetermined direction in a pixel 220, the orientations of the liquid crystal molecules look different from one another according to the viewing angle.
When viewed from the front left along a line 210, the liquid crystal molecules look as if they are aligned in a substantially horizontal orientation 212, and the image is relatively brighter. When viewed from the front along a line 230, the liquid crystal molecules are observed as being aligned in an orientation 232, which is the same as the actual orientation of the liquid crystal molecules inside the pixel 220. In addition, when viewed from the front left along a line 250, the liquid crystal molecules look as if they are aligned in a substantially vertical orientation 252, and the image is relatively darker.
Accordingly, the viewing angle of the LCD is greatly limited compared to other displays that spontaneously emit light since the intensity and color of light of the LCD varies according to a change in the viewing angle. In order to improve the viewing angle, a number of researches have been carried out.
FIG. 3 is a conceptual view illustrating a conventional approach to reducing a variation in contrast ratio and color shift depending on the viewing angle.
Referring to FIG. 3, a pixel is divided into two pixel parts, that is, first and second pixel parts 320 and 340, of which the orientations of liquid crystal are symmetrical to each other. Both the orientation of liquid crystal in the first pixel part 320 and the orientation of liquid crystal in the second pixel part 340 can be seen. The intensity of light reaching the user is the total intensity of light from the two pixel parts.
When viewed from the front left along a line 310, liquid crystal molecules in the first pixel part 320 look as if they are aligned in the horizontal orientation 312, and liquid crystal molecules in the second pixel part 320 look as if they are aligned in the vertical orientation 314. Then, the first pixel part 320 can look bright. Likewise, when viewed from the front right along a line 350, the liquid crystal molecules in the first pixel part 320 look as if they are aligned in the vertical orientation 352, and the liquid crystal molecules in the second pixel part 340 look as if they are aligned in the horizontal orientation 354. Then, the second pixel part 340 can look bright. In addition, when viewed from the front along a line 330, the liquid crystal molecules are observed as being aligned in orientations 332 and 334, which are the same as the actual orientations of the liquid crystal molecules inside the pixel parts 320 and 340. Accordingly, the brightness of the image observed by the user remains the same or similar even when the viewing angle changes and is symmetrical about the vertical center line of the image. This, as a result, makes it possible to reduce a variation in contrast ratio and color shift depending on the viewing angle.
FIG. 4 is a conceptual view illustrating another conventional approach to reducing variation in contrast ratio and color shift depending on the viewing angle.
Referring to FIG. 4, an optical film 420 having birefringence characteristics is added. The birefringence characteristics of the optical film 420 are the same as those of liquid crystal molecules inside a pixel 440 of an LCD panel and have the orientation symmetrical to that of the liquid crystal molecules. Due to the orientations of both the liquid crystal molecules inside the pixel 440 and the birefringence material of the optical film, the intensity of light reaching the user is the total intensity of light passing through both the optical film 420 and the pixel 440.
Specifically, when viewed from the front left along a line 410, the liquid crystal molecules inside the pixel 440 look as if they are aligned in the horizontal orientation 414 and imaginary liquid crystal molecules of the optical film 420 look as if they are aligned in the vertical orientation 412. The resultant intensity of light is the total intensity of light passing through both the optical film 420 and the pixel 440. Likewise, when viewed from the front right along a line 450, the liquid crystal molecules inside the pixel 440 look as if they are aligned in the vertical orientation 454 and the imaginary liquid crystal molecules of the optical film 420 look as if they are aligned in the horizontal orientation 452. The resultant intensity of light is the total intensity of light passing through both the optical film 420 and the pixel 440. In addition, when viewed from the front along a line 430, the liquid crystal molecules are observed as being aligned in orientations 434 and 432, which are the same as the orientations of the liquid crystal molecules inside the pixel 440 and the imaginary liquid crystal molecules of the optical film 420, respectively.
However, even if the approaches shown in FIGS. 3 and 4 are applied, color shift still exists according to the viewing angle, and thus color changes as the viewing angle increases.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.